As means for realizing an optical communication system with high capacity, a WDM optical communication system, in which plural optical signals with different wavelengths are multiplexed and propagated through a single optical fiber, is being actively investigated, and information transmitted therethrough is epochally increased.
In such a WDM optical communication system, it. is very important that a transmission distance is extended as long as possible and transmission levels of the respective optical signals are kept high and equalized.
The conventional WDM optical transmitter is composed of plural light sources for respectively generating optical signals with different wavelengths and a coupler for multiplexing them. The levels of the optical signals are respectively adjusted by controlling driving currents supplied to the laser diodes (LDs, hereinafter) in the light sources. Each of the optical signal levels is monitored at the output port of the coupler, and so adjusted that it becomes equal to the minimum value therein.
Another method for equalizing the output levels of the optical signals at the output port of the coupler is as follows. Variable optical attenuators are respectively connected with the output ports of the light sources, and the outputs of the variable optical attenuators are multiplexed by the coupler. First, each of the light sources is so adjusted that its output power becomes the maximum. Next, the variable optical attenuators are so adjusted that each of the optical signals at the output port of the coupler becomes equal to the minimum value therein.
Common disadvantage of both the aforementioned output control circuits in the conventional WDK optical transmitters is that the optical signal levels at the output port of the coupler are adjusted by attenuating the output powers of the light sources based on the minimum value therein, hence high transmitting optical signal levels cannot be obtained.